While an autologous vein remains the graft of choice, advanced vascular disease and prior surgical intervention limit the availability of autologous grafts. Therefore, the use of synthetic grafts provides a means for restoring blood flow to ischemic areas when no alternative is available. Over the past three decades, artificial grafts have been used to provide immediate restoration of blood flow to areas of ischemia as a result of atherosclerotic vascular disease. In addition, they have been used to provide vascular access for hemodialysis in patients with chronic renal failure, and in the repair of arterial aneurysms.
Although initially successful in restoring perfusion to ischemic tissues, the long term prognosis for these grafts is not encouraging. Commercially available grafts are far from ideal due to their inherent thrombogenicity. Over an extended period of time, grafts less than 4 mm in diameter lose their patency as they become occluded via fibrin deposition and cellular adhesion. This process appears to be secondary, and to be due in part to the thrombogenic nature of the nude, i.e. non-endothelialized, surface of an implanted prosthesis. See Berger et al., "Healing of Arterial Prostheses in Man: Its Incompleteness", Ann. Surg. 175:118-27 (1972).
Thus, much current research is being focused on lining prostheses with human endothelial cells, for producing a non-thrombogenic endothelial cell surface such as exists in native human vessels. In dogs, seeding of endothelial cells onto both small and large diameter grafts has been shown to result in a complete endothelial cell lining in one to four months. Since vascular endothelium is said to represent a unique non-thrombogenic surface, endothelial cells are reported to be "the first logical choice for lining small diameter vascular grafts." The transplantation of a functional endothelial cell lining onto the surface of a vascular graft has proven to increase patency rates and decrease thrombus formation on the flow surface in animal models.
Past and present studies have focused on the isolation of large vessel endothelial cells from vein segments, with the subsequent seeding of these cells on the graft lumenal surface. Tissue culture advances have also made the generation of large numbers of endothelial cells for high-density seeding on vascular prosthesis possible. These techniques have major drawbacks in the clinical setting. Endothelialization occurs at a slow rate, when low density seeding techniques are applied. High-density seeding, using cultured endothelial cells, requires the use of undefined media not easily applicable to the clinical setting.
It has been recognized that human microvascular endothelial cells i.e. the cells which are derived from capillaries, arterioles, and venule, will function suitably in place of large vessel cells even though there are morphological and functional differences between large vessel endothelial cells and microvessel endothelial cells in their native tissues. Microvascular endothelial cells are present in an abundant supply in body tissue, most notably in fat tissue, and may be used to establish a degree of implantation confluence prior to implantation, i.e. at least 50.degree., which dramatically improves the prognosis of most implants. For purposes of further description, fat tissue is designated as the exemplary source of microvascular endothelial cells, but it is recognized that endothelial cells from other tissues may be used as well.
To overcome the problems associated with seeding large vessel endothelial cells on prosthetic grafts, methods were developed for the isolation of microvessel endothelial cells from autologous adipose tissue, followed by high density seeding of a vascular prosthesis.
Although microvessel endothelial cells have been shown to be capable of endothelializing a blood-contacting surface, methods of procuring and depositing these cells in an operating room setting present special considerations. A vascular graft or other implant is treated to confluence using microvascular endothelial cells which are separated from fat, which is obtained at the beginning of the same uninterrupted surgical procedure. Fat tissue is removed from the patient after sterile conditions have been established. Microvascular endothelial cells in that fat area are then quickly separated from their related tissue by enzymatic digestion and centrifugation, and are used to treat a surface which is then implanted in the patient during the latter stages of the same operation. This procedure permits a patient to receive a graft which has been treated up to or above confluence with his own fresh endothelial cells.
The microvascular rich tissue obtained is perinephric fat, subcutaneous fat, omentum, or fat associated with the thoracic or peritoneal cavity. This tissue is then subjected to digestion using a proteolytic enzyme such as collagenase, comprising caseanase and trypsin, which is incubated with the tissue until the tissue mass disperses to produce a tissue digest. The microvascular endothelial cells are then separated from the digest using low speed centrifugation to produce an endothelial cell rich pellet. The pellet is washed with a buffered saline solution.
The resulting microvascular endothelial cells are then, preferably, suspended in buffered saline solution containing plasma protein, preferably about 1% plasma protein. This suspension, which comprises, on a volumetric basis, a pellet to solution ratio of 1:5 to 1:15, or preferably about 1:10, is then used to treat the surface by incubating cells with that surface until sufficient adherence of the microvascular endothelial cells to that surface occurs to provide at least 50.degree. confluence. As a result, an improved graft implant is provided having endothelialized surfaces which are either confluent, or which reach confluence quite rapidly (doubling within one population) following implantation.
Implants which can be treated to produce such an endothelial cell lining include but are not limited to, for example, intravascular devices such as artificial vascular prostheses, artificial hearts, and heart valves. The kit and methods of this invention for endothelializing surfaces can be used for surfaces comprised of known synthetic materials such as polyester, polytetrafluoroethylene (PTFE), or naturally occurring materials, such as umbilical vein, saphenous vein, and native bovine artery.
Methods currently used employ standard laboratory equipment such as beakers, flasks, centrifuge tubes, shaker baths, pipettes, syringes, and sterile hoods. In one method the donated tissue is immediately transferred to ice cold buffered saline (pH 7.4) wherein the buffering agent is preferably a phosphate, i.e. a phosphate buffered saline (PBS). The tissue is minced with fine scissors and the buffer decanted. The proteolytic enzyme collagenase, containing caseanase and trypsin, is added to the tissue and incubated at 37.degree. C. until the tissue mass disperses. The digestion occurs within 30 minutes and generally should be less than 20 minutes. The digest is transferred to a sterile test tube and centrifuged at low speed (700.times.g) in a table top centrifuge for 5 minutes at room temperature.
The pellet of cells thus formed consists of greater than 95% endothelial cells. These endothelial cells are described herein as microvascular endothelial cells (MEC) since they originate from the arterioles, capillaries and venule, all elements of the microvasculature. The MEC pellet is washed 1 time by centrifugation with buffered saline, preferably PBS. The MEC suspension is then, preferably, pelletized by centrifugation (200.times.g) and the pellet resuspended with protein containing buffer solution. Resuspension should be performed at a ratio of approximately 1:5 to 1:15 or about 1:10 volumes of packed microvascular endothelial cells to buffer solution. The cell suspension is added to tubular grafts and the ends clamped, or the cells layered upon the surface to be treated.
Optimum periods for cell interaction vary upon the material of the prosthesis, the nature of any pretreatments it may have received, and whether the surface of the prosthesis has been modified to improve its acceptance of the MEC. Following incubation for a sufficient time to permit adherence of the endothelial cells with the prosthesis surface, the surface is washed with a protein containing buffer.
In U.S. Pat. No. 4,820,626, and related applications, methods of treating a graft surface with endothelial cells are disclosed. According to those methods, subcutaneous adipose tissue is aspirated via a cannula and transferred by vacuum into a mucous trap. The trap is then transferred to a sieve inside a funnel which is placed in a sterile beaker. A rinsing solution is then poured over the tissue to remove red blood cells and lysed fat. The tissue is manually poured into a sterile Erlenmeyer flask containing collagenase solution and agitated at 370.degree. C. for 20 minutes. The collagenase slurry is manually poured into sterile conical centrifuge tubes, and spun for seven minutes at 700.times.g. The endothelial cells are then pipetted out of the tube. A graft is tied to a male luer extension, and secured within a tube. The cells are resuspended in serum protein media and drawn into a syringe using a needle and syringe. The cells are forced into the lumen of the graft. The graft is manually rotated for 2 hours.
In spite of these advances, a need still exists for a simple, reliable method for producing endothelial cell coatings on a graft in an operating room setting, and this invention solves that need. The present invention provides for the ready isolation of large quantities of endothelial cells which can be easily performed in an operating room. While endothelial cells can be isolated from tissues other than fat, such as brain, lung, retina, adrenal glands, liver and muscle, the use of fat tissue as the source of the cells is preferred due to its abundance and availability, and due to the fact that its removal should not adversely affect the patient being treated. Although less preferred, it is possible to obtain human perinephric fat from brain-dead but heart beating cadaver donors, or from donors other than the donor's surgery. The isolated endothelial cells are then deposited on a graft for implantation.